Trying to think of or understand the rationale behind this change. Could the rendering be more exact using a Simulator? If so, and I doubt it would be, does that really matter? Do we all use this basically to test breakpoints?
Regardless, they have done this to lighten the workload of the Safari developer tools team (let's say between 1 and 100 people, bit i have no idea) and have instead decided to waste the time, energy, bandwidth and electricity of every user of this feature across the entire world.